The present application concerns a method and apparatus for use in the location of elongate conductors, particularly buried conductors, e.g. pipes, cables or other utilities. Instruments currently being marketed can measure the alternating magnetic field due to a buried long conductor that carries alternating current, by measuring currents induced in one or more xe2x80x9csearchxe2x80x9d coils. Depending on the particular instrumental arrangement, maximum or null magnetic field and induced current should indicate that a conductor is directly beneath. However the instruments are unable to distinguish individual conductors, within a combined field resulting from several disparate conductors. Where other conductors lie close to that sought, their currents create a combined field that can give misleading results and lead to false deductions as to conductors"" positions. In practice, quite frequently, underground plant or general site safety is imperilled by such inaccurate findings.
The present invention makes use of the fact that if a coil whose axis is vertical is situated directly over a remote (typically buried) long conductor, it will receive zero induced current from an alternating electric current flowing in that conductor. This is regardless of the current strength in the conductor or the distance between it and the coil. It is this phenomenon that it is proposed be used in the manner of an electrical balance device. On either side of the conductor the induced current in a coil increases equally, but in opposed phase, positively to the right, if negatively to the left, and vice-versa.
The induced current received by a coil of vertical axis is the product of two influences. Firstly, it varies according to the inverse of the distance between the coil and the conductor (measured from the centre of the coil). Secondly, it varies as the cosine of the angle between a horizontal line joining the coil axis (at the coil""s centre) to the nearest point vertically above the conductor, and a line joining the coil axis (at the coil""s centre) to the conductor axis, perpendicular to the conductor axis. Thus the second factor, and the product of the two factors, are zero where the coil is above the conductor.
According to the invention in a first aspect there is provided an apparatus for use in the location of elongate horizontally extending conductors comprising an antenna comprising a support structure and an array of five substantially identical coils mounted thereto, in a horizontal row, each coil having a linear axis, the coils being arranged so that their axes are vertical and coplanar and with equal linear spacings. Generally the five coils of said array are connected to a signal processing unit so that signal currents induced in the coils can be input to said unit simultaneously for processing thereby.